We intend to study at the ultrastructural level, employing both scanning and transmission electron microscopy, the morphological steps or events of specifically induced in vitro macrophage-macrophage fusion which results in giant cell formation and to investigate morphologically the non phagocytic death of tumor cells resulting from macrophage-tumor cell fusion. We inted to pursue preliminary observations of a morphologically subpopulation of alveolar cells. We inted to isolate, partially purify and characterize the soluble BCG-sensitized lymphocyte product termed macrophage fusion factor (MFF) which has been reported to be responsible for macrophage-macrophage fusion and multinucleated giant cell formation. The role of MFF in the well documented but insufficiently studied non-phagocytic death of tumor cells through macrophage tumor cell fusion in vitro and in vivo shall be ascertained.